1. Field of the Invention
The invention relates generally to differential amplifiers and, more specifically, to clamps for current feedback differential amplifiers.
2. Description of the Related Art
Voltage feedback amplifiers are used in applications in which high linearity and low distortion are primary requirements. However, voltage feedback amplifiers are inherently slow in operation due to a requirement for charging or discharging stray capacitances at circuit nodes in order to change the voltage at those nodes.
Current feedback amplifiers are capable of much faster operation than typical voltage feedback amplifiers since the charging current available for charging and discharging the capacitances at circuit nodes can be significantly greater. Other well known advantages of current feedback amplifiers include fast slew rates and fixed bandwidth vs gain. Two key disadvantages of current feedback amplifiers are related to their low impedance feedback input node with its intrinsic linearity limits and excessive shot current noise.
A conventional current feedback amplifier may include a class AB push-pull input stage in the form of a transconductance amplifier as disclosed in U.S. Pat. No. 4,780,689, entitled "AMPLIFIER INPUT CIRCUIT", issued Oct. 25, 1988 to K. Saller et al. Two such A/B cells can be used as the front end of a differential feedback amplifier, as illustrated in U.S. Pat. No. 5,410,274, entitled "SINGLE-ENDED AND DIFFERENTIAL AMPLIFIERS WITH HIGH FEEDBACK INPUT IMPEDANCE AND LOW DISTORTION", issued Apr. 25, 1995 to Dwight Birdsall et al.
Cancellation of common mode modulation errors introduced in input stage amplifiers has been accomplished by cross-connecting the back end amplifier outputs with the front end inputs through resistive feedback loops and interconnecting the front end amplifier outputs through a resistor. The overall amplifier has low shot noise since both of its voltage input terminals have high impedance,is fully differential, and achieves the advantages of voltage and current feedback.
High impedance clamps have been connected to the inputs of the back end amplifiers so that, whenever the front end input signals exceed a threshold, the clamps limit the signals at the inputs to the output amplifiers, thus ensuring that these amplifiers are not overdriven. However, the backend outputs are also clamped and the feedback loop is effectively opened. As a result, any further increase in the input signal appears across the resistor connecting the outputs of the front end amplifiers. Since this resistor is generally between 5 .OMEGA. and 10 .OMEGA., the resulting current could be quite large and may damage or destroy the front end amplifiers.